1. Field of the Invention
The present invention relates to rotary drill bits, and more particularly to drill bits having a plurality of rotary conical elements with cutting teeth thereon for drilling oil wells and the like.
2. History of the Prior Art
Rotary drill bits, sometimes called rock boring bits, are commonly used in the drilling of oil wells and for other ground boring requirements. Such drill bits typically employ a plurality of rotary conical elements having hardened metal tips or cutting teeth on the surface thereof. Such roller cutters or cones are rotated under the weight of a drill pipe to which the drill bit is coupled. This forces the drill bit into a rock or other ground formation, and the rotation causes the rotatably mounted cones to rotate about their own axes. The cutting teeth on the surfaces of the cones chip and crush the rock or other formations.
An example of a rotary drill bit is provided by U.S. Pat. No. 4,393,948 of Carlos Fernandez, which patent issued Jul. 19, 1983 and is entitled "Rock Boring Bit With Novel Teeth And Geometry". The Fernandez patent describes a drill bit in which a body secured to a pin has a plurality of legs mounted in equally spaced keyways therein. Each leg has a threaded support pin on an end thereof opposite the body for rotatably mounting a roller cutter in the form of a cone having a plurality of cutting teeth on the outer surface thereof. The cone is rotatably mounted on the support pin by an arrangement including a thrust plate, a pair of roller bearings and an arrangement of ball bearings. A circular seal disposed at the underside of the cone base cooperates with the leg on which the cone is mounted.
In the cones of drill bits according to the Fernandez patent, the cutting teeth are arranged in rings which extend around the central axis of the cone, with each of the rings being axially spaced along the axis and overlapping with at least one other ring and having the cutting teeth of the respective rings interspersed. The cutting teeth have edges which are disposed in oblique relationship to the axis of the cone. The edges within any one ring are disposed in oblique relationship to the rectilinear edges of at least a plurality of other cutting teeth proximate thereto.
Presently known drill bits suffer from a number of disadvantages. In conventional drill bits, the legs which mount the cones are typically angled relative to the central axis of the drill bit so as to form an outwardly extending array from the body. Such arrangement, however, is not without its limitations in terms of difficulties that may be encountered in making and assembling such a drill bit, or from the standpoint of attempting to design the drill bit for use with legs of different size. Furthermore, such conventional drill bits typically mount a plurality of mud spraying nozzles at the top of the body in an array which is not especially efficient. The body is typically bolted to a threaded pin which in turn is used to couple the drill bit to a drill pipe. As the drill pipe is rotated, substantial torque is exerted on the body with its included legs and cones, and much of this torque is in turn exerted on the body-pin interface.
Nozzles for spraying wash water or mud in conventional drill bits typically have apertures of fixed size for determining the amount of the mud spray therefrom. Because it is customary to provide a drill bit with nozzles having different aperture sizes, it is frequently necessary that an inventory of different size nozzles be available. In addition, because such nozzles are typically made of hardened metal, they experience wear with usage and must periodically be replaced as the wear becomes great enough to alter the fixed aperture size. For air drilling, typically the nozzles are removed so that air under pressure is simply blown through the apertures in which the nozzles are mounted.
In conventional drill bits, as exemplified by the drill bit described in the previously referred to U.S. Pat. No. 4,393,948 of Fernandez, each cone is typically held on the leg on which it is mounted by an arrangement of ball bearings disposed within opposite races in the leg and in the underside of the cone. Particles of crushed rock, sand and other debris eventually enter the area between the cone and the leg and score or wear the bearings and bearing races. Eventually, the cones become separated from the legs, and are usually lost.
In conventional drill bits, provision is usually made for accommodating the thrust loads which result from the cone being pushed against the threaded support pin or spindle of the leg on which the cone is mounted. Various different thrust bearings, thrust plates or similar devices are used in an effort to accommodate the thrust loads. One common technique involves the use of thrust buttons. Such buttons are of limited size and are further limited in terms of the compression loads which can be accommodated. Other thrust bearing arrangements in conventional drill bits have similar limitations.
In certain types of conventional drill bits, a grease reservoir is provided. In the event of failure of an o-ring seal, the grease reservoir acts to inject additional lubricating grease into the bearing so as to maintain the drill bit functional for an additional period of time before ultimate failure occurs.
In conventional drill bits, the cones are typically formed using an expensive forging process. Thereafter, the cutting teeth are machined, one at a time. A hard facing material is then welded over the cutting teeth. The high temperatures involved in the welding process tend to reduce the metallic core hardening of the metal.
In conventional drill bits, gaging of the hole being drilled by the bit is typically accomplished solely by a row of cutting teeth at the outer peripheries of the cones. Such cutting teeth extend from an outer rim formed at the base of the cone. Because such rows of cutting teeth perform a gaging function to the exclusion of other backup components, they tend to wear rapidly, and this eventually exposes the cones to wear and damage.
In conventional drill bits, the bearings at the cone-leg interface are sometimes sealed using o-rings or similarly shaped seals. Such seals are typically secured to either the leg or the cone so as to undergo sliding contact with the other as the cone rotates. As a result, considerable friction is generated between the seal and the sliding surface which contacts the seal, and this results in the generation of considerable heat. Consequently, such seals tend to wear out rather quickly.
In many conventional drill bits, the central axis of each cone is offset in order to produce a slippage or scraping action in the bottom of the hole being drilled. Rows of parallel edges of the cutting teeth are located at different distances from the center of the hole, and overlap. However, there remains a gap between rows of cutting teeth where no crushing action occurs. Consequently, the three cones of a drill bit together may not crush more than about 85% of the total surface of the bottom of the hole. Manufacturing tolerances and different numbers of cutting teeth at different locations, from cone to cone, result in different torques and a consequent wobbling action. Furthermore, only one side of the cone exterior typically touches the bottom of the hole. As a result, the cones wobble and the holes being created are not straight but are more spheroid in shape. This allows the drill pipe to rub against the hole wall, requiring more driving power. Also, when the formation being drilled softens, it is not possible to increase the bit load due to a loss of direction.
Further shortcomings of the cones typically used in conventional drill bits include the tendency of the cutting teeth to enter the ground from the side thereof, which tends to bend or break the teeth. The tendency of the cutting edges of the teeth to scoop straight ahead further adds to the difficulty in drilling. Also, the large gauging teeth at the base of the cone tend not to enter the ground at the bottom edges of the hole in a very effective manner.
Conventional drill bits typically use grease as the lubricant. The use of grease is preferred in such drill bits which typically are not sealed at all, or at best are provided with some sealing action which is somewhat unreliable. However, the grease is difficult to distribute uniformly throughout the bearings and is very difficult to circulate.
In conventional bearings which utilize bushings, the bushings are typically installed in an aperture in the bearing with either a press fit or a slip fit. The slip fit is often regarded as being advantageous, inasmuch as it allows the bushing to rotate within the aperture, with a consequent reduction in friction and the heat buildup which results therefrom. Where a slip fit of the bushing is used, however, there is usually no way of determining whether the bushing is turning within the aperture or when it may have become stuck within the aperture due to such things as heat expansion.